Halovinyl polymers such as polyvinyl chloride , poly-(vinylidene chloride) are used in the construction industry. Halo-vinyl polymers are subjected to post-polymerization halogenation to improve its properties such as mechanical and dielectric characteristics, and resistance to acid, oil and gasoline. Chlorinated poly-vinyl chloride (CPVC), chlorinated poly-(vinylidene chloride) (CPVDC), chlorinated ethylene-chlorotrifluoro-ethylene copolymer (CE-CTFE), chlorinated poly-propylene and chlorinated poly-ethylene are some of the well-known examples of halogenated polymers. Chlorinated vinyl chloride polymers with high mechanical and dielectric characteristics, and resistance to acid, oil and gasoline are widely used as construction materials.
Chlorinated polyvinyl chloride (CPVC) has better ductility, greater flexure and crush resistance properties than polyvinyl chloride (PVC). It can withstand corrosive water at higher temperatures than PVC, typically 40° C. to 50° C. (104° F. to 122° F.) or higher. Further, it also exhibits better non-flammable property as it is very difficult to ignite, and tends to self-extinguish in a directly applied flame. It is ideally suited for self-supporting constructions where temperatures up to 200° F. (90° C.) are required. CPVC is therefore suitable for replacement of various metals used for the preparation of pipes in corrosive conditions.
Products made by CPVC exhibit the desired properties, but have serious limitations in terms of stability and effective life. Such limitations are highly attributed to the residual acidity of the polymer resin. It has been observed that the desired stability and life span of the CPVC products can be enhanced if the acidity of the CPVC resin is maintained at or below 2000 ppm. Acidity of the chlorinated polymer resin is due to the residual chlorine and hydrochloric acid.
Various physical and chemical treatments for the removal of residual chlorine and hydrochloric acid from the polymers have been reported. Physical treatments include centrifugation, rotary drying, microwave treatment, vacuum drying, vacuum distillation, ultra-sonication, hot water treatment, passing warmed nitrogen gas and any combinations thereof. The chemical treatments generally include neutralization, using bases such as alkali metal salts of weak acids, alkaline metal salt solution, strong base, ammonium hydroxide, hydrogen peroxide, sodium hypochlorite, zeolites, polyatomic oxides and hetero-polyacids. Some processes involve a physical treatment such as centrifugation of the polymer resins followed by a chemical treatment such as treatment of the centrifuged polymer resins with bases, e.g., sodium hydroxide and sodium carbonate.
EP599795 mentions use of calcium hydroxide to trap hydrochloric acid evolved during the decomposition of polyvinyl chloride.
U.S. Pat. No. 4,730,035 mentions a process for reducing the quantity of hydrochloric acid to 266 ppm of the CPVC resin by passing dry gas through a wet CPVC resin in the presence of calcium stearate solution.
U.S. Pat. No. 5,359,011 mentions use of bases like sodium citrate and sodium borate for effective neutralization of hydrochloric acid, which gives a stability value of 60 to 80 seconds for CPVC polymer in hydrochloric conductivity test.
U.S. Pat. No. 0,063,247 mentions a process for hydrochloric acid removal by chemical treatment using peroxides and hypo-chlorites.
U.S. Pat. No. 7,345,114 mentions neutralization of the acidic liquor during chlorination using sodium hypochlorite. However, the polymer, shows light yellow colour after neutralization.
U.S. Pat. No. 4,386,189 mentions removal of residual chlorine and hydrochloric acid from chlorinated polymer resins by first subjecting it to a physical treatment followed by a chemical treatment. The physical treatment includes purging nitrogen gas through the resins to expel out chlorine from the polymer resin. The chemical treatment includes neutralization of the acidic liquor by contacting the polymer resins with suitable bases.
However, in the mentioned processes it is observed that after subjecting the chlorinated polymer to physical treatment, still a substantial amount of chlorine and hydrochloric acid remains trapped in the pores of polymer resins. This trapped residual chlorine and hydrochloric acid gets released during chemical treatment in the form of hydrochloric acid, which in turn requires neutralizing agents in large quantities. Therefore in these processes, chemical treatments are found to be more time consuming, resource intensive and less effective. Further, higher concentration of a neutralization medium increases the cost of the process and the hazardous ingredients in the effluent stream, and also increases the risk of deterioration of the texture of the polymer resins. Further, the exposure of the polymer resins to a single treatment for a longer time increases the possibility of de-hydrochlorination of the polymer resins. For example, the chemical treatment of CPVC polymer using an alkaline medium such as sodium hydroxide or sodium carbonate solution is found to be very effective for neutralization of hydrochloric acid if carried out for a short period, but with longer time period the risk of de-hydrochlorination or undesired coloration of the polymer resin increases. Similarly, structural defects can also evolve on long exposure of the polymer reins to physical treatments such as treatment with hot water, which gives a dark coloration to the resin. Some of the chemical treatment methods also requires costly chemical for effective neutralization of residual acidity which increases cost of the process.
Therefore, there exists a need for a simple, economic and environment friendly process which can effectively reduce residual chlorine and hydrochloric acid from the halogenated polymer without affecting the structure and stability of the polymer within a comparatively lesser time period, using mild chemical treatment conditions than the conventional post-chlorination processes.